950 jockey
hi, yeah change them both. like the headlights on your car,,,,change the left then the right dies a week later!
you will be fine running 10 or 11 tooth with 9 speed chain.just keep the diameter similar to what you have now and cleaarnce will/should be ok
the xt was 'mega 9' if i remember rightly :? but was used in a 27 speed set-up with great success.
some 7 and 8 speed jockey wheel were narrow anyway, remember the shimano 'centeron'? you have probably handled a few from mid- nineties LX and suchlike.i would avoid those large sram type and 'rollerboys' to be on the safe side.
let us know how you get on,mon aimee.